JP08-219004A, published by the Japan Patent Office in 1996, proposes a driving device for a swash plate hydraulic motor used to generate travel power for a hydraulic shovel.
Referring to FIG. 8, this driving device comprises a motor capacity change-over actuator 93 that changes a tilt angle of a swash plate of a hydraulic motor 91 and a motor capacity change-over valve 95 that changes a working fluid pressure for driving the motor capacity change-over actuator 93.
In a high speed position X, the motor capacity change-over valve 95 supplies a pressurized working fluid in a high pressure port 94B to the motor capacity change-over actuator 93. The motor capacity change-over actuator 93 is driven to expand by the pressurized working fluid such that the tilt angle of a swash plate 92 of the hydraulic motor 91 decreases. As a result, a rotation speed of the hydraulic motor 91 increases.
To decelerate the hydraulic motor 91, the motor capacity change-over valve 95 is changed over from the high speed position X to a low speed position Y. In the low speed position Y, a tank port 94C communicates with the motor capacity change-over actuator 93. The motor capacity change-over actuator 93 is operated by a reactive force from the swash plate 92 to contract while discharging the working fluid to a tank 100. As a result, the tilt angle of the swash plate 92 of the hydraulic motor 91 increases, leading to a reduction in the rotation speed of the hydraulic motor 91.
A flow control valve 98 constituted by a fixed orifice 96 and a pressure reducing valve 97 is provided between the motor capacity change-over valve 95 and the tank port 94C.
The flow control valve 98 keeps a flow rate of the working fluid that is discharged from the motor capacity change-over actuator 93 to the tank 100 via the tank port 94C during deceleration of the hydraulic motor 91 substantially constant. By keeping a contraction operation speed of the motor capacity change-over actuator 93 constant using the flow control valve 98, a shock generated as the hydraulic motor 91 decelerates is alleviated.